1. Field of the Invention
The present invention relates to a facile synthetic method of a series of 2-(4′-alkylphenyl)-5-cyanopyridine liquid crystal compounds, and more particularly to a synthetic method which uses the Grignard reagent and promotes electrophilic properties of pyridines in the synthesis.
2. Description of the Prior Art
With the rapid development of internet, communication and computer manufacturing technologies, a variety of portable electronic products such as cellular phones, mobile computers, personal digital assistants (PDA), palm audio/visual players (e.g. MP3 and MP4 players), and the like, are playing an indispensable role in modern life. As the present electronics moves in a trend of thinner products, liquid crystal displays have been gradually taking the place of conventional cathode ray tubes. Because of their advantages in thin structure, light weight, low power consumption, low radiation contamination and compatibility with semiconductor processing technology, liquid crystal displays have found broad applications in diverse electronic products. In order to advance liquid crystal manufacturing technology, it is essential to further improve and develop liquid crystal materials.
Due to its advantages in high response speed, high resolution and wide viewing angle, the Highly Sensitive Electric Field Induced Liquid Crystal has become a common optical switching material used in the new generation of liquid crystal displays. Among the Highly Sensitive Electric Field Induced Liquid Crystals, 2-(4′-alkylphenyl)-5-cyanopyridine liquid crystal compounds are similar in molecular structure to 4-cyano-4′-pentyl biphenyl (5-CB), which has been widely used in commerce. Therefore, the former has a great potential to be used as molecular optical switches in liquid crystal displays.
Conventional synthesis of 2-(4′-alkylphenyl)-5-cyanopyridine was disclosed in JP H04-213386, as shown in FIG. 1. First, sodium methoxide CH3ONa (12), ethyl formate (HCO2C2H5) (13) and cyanoacetamide (CNCH2CONH2) (14) are added to acetophenone (11) for cyclization (yield 38-42%) to form 2-cyano-5-(4′-alkylphenyl)pyridone (15), followed by the addition of phosphorus oxychloride (POCl3) (16) for aromatization (yield 70-78%) to form 2-(4′-alkylphenyl)-5-cyano-6-chloropyridines (17). Finally, zinc (Zn) (18) is added for dechlorination (yield 39-70%) to form the liquid crystal compound (I) of 2-(4′-alkylphenyl)-5-cyanopyridines. The abovementioned synthetic method with three steps involves many procedures and consumes a large amount of reagents; moreover, some of the reagents are costly and, worst of all, the total yield is only 10-23%. For these reasons, the method is not suitable for industrial production.
Accordingly, the aforementioned synthetic method of 2-(4′-alkylphenyl)-5-cyanopyridine liquid crystal compounds still has many disadvantages and needs to be improved.
The inventor, in view of the drawbacks of said conventional synthetic method of 2-(4′-alkylphenyl)-5-cyanopyridine liquid crystal compounds, after many years of R&D on innovative improvement, has successfully developed a facile synthetic method of a series of 2-(4′-alkylphenyl)-5-cyanopyridine liquid crystal compounds.